In recent years, as a power storage system, a sodium-sulfur battery (hereinafter, referred to as a NaS battery), which is a secondary battery using a reaction between sodium and sulfur, has been under development.
Such a NaS battery has a structure in which, for example, a β-alumina solid electrolyte tube is charged in a positive electrode container made of aluminum or the like, sulfur is loaded into a space between the positive electrode container and the β-alumina solid electrolyte tube, and sodium is loaded into the β-alumina solid electrolyte tube.
In addition, a NaS battery has a constitution in which, for example, the ion conductivity of a β-alumina solid electrolyte is increased in a high-temperature environment of 300° C. or higher so as to cause sodium and sulfur to reversibly react with each other through the solid electrolyte, thereby repeating charging and discharging. Furthermore, the positive electrode container and the β-alumina solid electrolyte tube are, generally, housed in an exterior container made of stainless steel or the like for the convenient handling thereof.
Since the NaS battery deteriorates as charging and discharging are repeated, the NaS battery has a service life of approximately 10 years, and it is necessary to appropriately treat the NaS battery that has been used for the entire service life. Particularly, since metallic sodium generated in the charged NaS battery reacts with moisture and thus easily generates hydrogen, it is necessary to appropriately treat the NaS battery. Therefore, for example, Patent Documents 1 to 3 disclose a variety of techniques for treating NaS batteries that have been used for the entire service life.
For example, Patent Document 1 discloses a method in which the NaS battery is cut open at the mouth and is immersed in a heating oil tank, and molten sodium is caused to flow down from the inside and is recovered.
In addition, Patent Document 2 proposes a method in which a NaS battery that has been used for the entire service life is disassembled and molten sodium is removed into a paraffin tank and a NaS battery having a structure in which the NaS battery is easily disassembled.
Furthermore, Patent Document 3 proposes a method in which a NaS battery that has been used for the entire service life is cut open at an electrode cap, the positive electrode container or the solid electrolyte tube is broken, and the NaS battery is incinerated in excess air.